Corrosion resistant steels having improved weldability

ABSTRACT

A CORROSION RESISTANT STEEL HAVING IMPROVED WELDABILITY CONSISTS OF 0.001-0.30 PERCENT CARBON, 0.1-2.0 PERCENT SILICON, 0.3-2.0 PERCENT MANGANESE, 0.01-0.50 PERCENT CHROMIUM, 0.1-0.29 PERCENT COPPER, 0.00010.040 PERCENT PHOSPHORUS, TWO OR MORE ELEMENTS SELECTED FROM THE GROUP CONSISTING OF ARSENIC, BERYLLIUM, BISMUTH, LEAD, GERMANIUM, ANTIMONY, SELENIUM AND TELLURIUM IN AN AMOUNT O 0.02-0.20 PERCENT EACH, BALANCE BEING IRON AND UNAVOIDABLE IMPURITIES. THE STEEL MAY ALSO INCLUDE ONE OR MORE ELEMENTS SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM, NIOBIUM, VANADIUM, MOLYBDENUM, TUNGSTEN AND NICKEL IN AN AMOUNT OF 0.010.50 PERCENT IN RESPECT TO TITANIUM, ZIRCONIUM, NIOBIUM AND VANADIUM, 0.1-1.5 PERCENT IN RESPECT TO MOLYBDENUM AND TUNGSTEN AND 0.1-3.5 PERCENT IN RESPECT TO NICKEL, BALANCE BEING IRON AND UNAVOIDABLE IMPURITIES.

y 15, 1973 TADASHI NISHI 3,733,195

CORROSION RESISTANT STEELS HAVING IMPROVED WELDABILITY 2 Sheets-Sheet 1 Filed Jan. 14, 1970 r r C r zorc C2 0 n 2 0 M0. T 5 0 n M M 93m nl|8 8 N O 6 O l Q My 0 5 lo M M [F o O O O O RELATION BETWEEN Cr AND Si CONTE NTS AND CORROSION RATE INVENTORS M mmm AAOOAA AAUr A IWMS MK N m O HA .IHIH SY KS A U E fi DNMRMMM MwHmTK ATTORNEYS 15, 1973 TADASl -H NISHI ET AL 3,733,195

CORROSION RESISTANT STEELS HAVING IMPROVED WELDABILITY Filed Jan. 14, 1970 2 Sheets-Sheet 2 'Ooo CONVENT lo NO CRACKING IONALSTEEL A CRACKINGE IZ A IN [x CR CK G INVENTION A IN P STEEL 0 NO CR CK G 2 g soo 6 13 (I D U 2 7oo- Q Q 1; 1 0 I u.|

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50o I Q A 30 4o 50- so 70 WELDING SPEED FIG.3

ATTORNEYS United States Patent 3 733 195 CORROSION RESITAiNT STEELS HAVING IMPROVED WELDABILITY Tadashi Nishi, Tuneyasu Watanabe, Haruo Shimada,

celeration of corrosion, electrolytic aqueous solution, such as sea-water and plain water contained in the crude oil plays an important role. Further acceleration of corrosion is efiected by presence of sludge and bacteria in the crude oil.

Hiroki Masumoto, Takeshi Fujirnoto, Kazuhiro Miida, 5 and Takashi Okazalki, Kitakyushushi, Japan, assignors In such manna applwat1ons as P f corfosmn to Nippon steel CorPomtionTokYo, Japan of steel materials is due to the physiological action of Fil J 14, 1970, S 2, 55 sulphate-reducing bacteria during the reduction of sul- Claims priority, application Japan, Jan. 16, 1969, phate ions contained in the sea-water into sulphur, and i l/3,003; Feb. 28, 1969, A t/15,6150 10 thus the corrosion mechanism is quite different from Czzc 39/ 54 those of ordinary weather-resistant steel materials (par- 75-125 2 Chums ticularly corrosion by industrial atmospheres) and is characterized by severe local corrosion and pit corrosion. ABSTRACT OF THE DISCLOSURE This is the very reason why steel materials having only A corrosion resistant steel having improved weld- 15 weather-resistance alone cannot be used as sea-water reability consists of 0.001-030 percent carbon, 0.1-2.0 sistant steel materials. percent silicon, 0.3-2.0 percent manganese, 0.01-0.50 Presently, cathodic protection by using galvanic anode percent chromium, 0.1-0.29 percent copper, 0.0001- drainage using zinc, aluminium, magnesium etc. and cor- 0.040 percent phosphorus, two or more elements selected rosion inhibitors have been proposed for preventing such from the group consisting of arsenic, beryllium, bismuth, 0 types of corrosion as above. But, all of these methods are lead, germanium, antimony, selenium and tellurium in only temporarily elfective and require very large cost an amount of 0.02-0.20 percent each, balance being and labour for maintenance. iron and unavoidable impurities. The steel may also in- Further, steels as shown in Table 1 have been proclude one or more elements selected from the group conposed for such marine applications, which are improved sisting of titanium, zirconium, niobium, vanadium, in sea-water resistance as well as in mechanical propmolybd-enum, tungsten and nickel in an amount of 0.01- erties by addition of phosphur, copper, chromium, 0.50 percent in respect to titanium, zirconium, niobium and aluminum.

TABLE 1 Percent of- Others 0 Si Mn P S Cu Cr (percent) so. 20 50.50 50. 00 0.07-0.15 50.01 0.02-0.11 0.2-0.8 50.22 50.10 0.0-0.9 0.08-0.15 0.04 20.5 Ni:0.4(H).65 50.18 50.50 30.0 0.08-0.15 0.03 0.6-1.5 0.5-3.0 Al:0.51.5 50.13 50.50 50.5 30.03 $0.025 3.0-4.3 Al:0.71.1

and vanadium, 0.1-1.5 percent in respect to molybdenum The above conventional sea-water resistant steels are and tungsten and 0.1-3.5 percent in respect to nickel, improved in sea-water resistance chiefly by the addition balance being iron and unavoidable impurities. of phosphorus and/or copper, but these additives present a critical defect that cracks at welded portions are remarkably increased in welded structures such as sub- BACKGROUND OF INVENTION marine pipe lines and buoys. Conventional sea-water resistant steels have the follow- Further, many trials have been proposed for improving corrosion problems. ing sea-water resistance of steels by the addition of special (1) In case of marine structures such as base piles elements, For example, Hudson teaches in Journal of and buoys which are exposed and splashed with sea- Iron and Steel Institute (JISI), 1955, July, vol. 180, water the corrosion of portions splashed with sea-water pp. 271-284t that chromium, phosphorus, silicon, alumiis as high as 3 to 7 times the corrosion of other pornum, molybdenum, nickel and beryllium are useful while tions (portions that are in the sea-water above sea), tin, antimony arsenic, bismuth, lead, tungsten, niobium the corroded surface conditions are characterized by and vanadium are not useful. severe local corrosions and pin corrosions, and the cor- However, Hudsons findings are limited to results obrosion products (rust) of the splashed portions have retained from studies on corrosions in sea-water and in markable tendency of sticking as compared with the plain water, and does not teach of corrosions caused other portions. by repeated splashing of the sea-water or corrosions under (2) In case of applications such as marine steel piping a condition where both crude oil and sea-water are presand floats which are immersed always in the sea-water, cut. The type and mechanism of corrosion under such severe corrosion takes place particularly on portions of conditions are quite different from those in sea-water or the structure which are 1-3 m. below the sea surface. in plain water. This is due to the physiological action of sulphate-reducing bacteria which reduces sulphate ions in the sea- DESCRIPTION OF INVENTION water into sulphur and corrode the steel remarkably. The present invention relates to corrosion resistance This action is greater in contaminated zones and Warm steels, particularly sea-water resistant steels, having imdistricts of the sea. proved weldability, useful for applications such as ship (3) In case of applications such as oil tankers, oil hulls, buoys, landing piers, base piles, dolphins (marine pipe lines which are exposed alternately or simultaoil drilling platforms) which are exposed to splashing neously with the sea-water and oils. of sea-water; such as marine or submarine steel pipes and The corrosion is a complicated combination of wholefloats which are always immersed in the sea-water; and surface corrosions and local corrosions, the corrosion rate such as parts of ship hulls and oil pipe lines which are of the latter type being extraordinarily high. In general, corrosion in crude oil alone is very small, and for acexposed alternately or simultaneously to oils, sea-water and plain water.

The basic composition of the present inventive steels comprises 0.001-0.25 percent by weight of carbon 0.1- 2.0 percent by weight of silicon, 0.3-2.0 percent by weight of manganese, 0.01-3.0 percent by weight of chromium, 0.1-0.29 percent by weight of copper, less than 0.044 percent by weight of phosphorus and the balance being iron and unavoidable impurities (hereinafter percent by weight is abridged as The present invention will be described referring to the attached drawings in which:

FIG. 1 is a graph showing effects additions of silicon, manganese and chromium on corrosion rates in a seawater splashing zone. FIG. 2 is a perspective view of a test piece used for the weld-crack test in Example 3, and FIG. 3 is a graph showing results of the weld-crack tests on test pieces such as shown in FIG. 2 obtained from the conventional steels and inventive steels.=

One type of a preferred composition of the present inventive steels comprises: 0.00l-0.25% of carbon, 0.8- 1.l% of silicon, 03-20% of manganese, ODS-3.0% of chromium, 0.l-0.29% of copper, less than 0.04% of phosphorus, the balance being iron and unavoidale impurities, in which the phosphorus and/or copper content in the conventional steels have been substituted by a combined effect of Si-Mn-Cr for eliminating welding cracks and improving sea-water resistance.

In the above preferred type of composition, reasons for limitation of each of the elements are as follows:

Silicon and manganese are well known as elements necessary for deoxidation in steel production, and chromium and copper are well known as elements necessary for improving strength and weather-resistance of steels.

However, in the present invention, when these elements are present in suitable amounts, a remarkably improved corrosion resistance which cannot be expected from the addition of one of these elements can be obtained.

For example, when both silicon and chromium are present in the steel, corrosion in a contaminated seawater is remarkably reduced. This effect increases as the content of silicon and chromium increases, thus reducing the corrosion, including pit corrosion.

Particularly, an increased content of silicon is effective for forming a silicate film on the surface of the corrosion product (metal oxides) and is thus effective for preventing corrosion action of the sulfate-reducing bacteria in the sea-water.

However, as shown in FIG. 1, it is desirable in the sea-water splashing zone where the amount and rate of COITOSiOl'l are large that the contents of both silicon and manganese are high while the content of chromium is low. The corrosion in this sea-water splashing zone is not stabilized in spite of a similar silicate film as above, but rather accelerated by a potential difference (or local electrode effects) between the base metal and the carbides of manganese and chromium formed by the corrosion. Therefore, the content of manganese is increased to reduce the corrosion or the content of chromium is lowered to prevent the carbide formation.

The present inventive steel having a composition as above shows an excellent sea-water resistance in the contaminated sea zone or in the sea-water splashing zone, and the proportion range of each element is defined as above because a steel having a composition in the above range is useful from the points of corrosion resistance, manufacturing, mechanical properties and weldability.

Namely, carbon is an element necessary for required strength of the present inventive steel. However, more than 0.25% of carbon causes embrittlement of the steel and lowers weldability and sea-water resistance, while less than 0.001% of carbon lowers steel strength and gives poor economy and productivity caused by a longer refining time. Silicon is necessary for deoxidation in the steel production, and 0.8-1.1% of silicon in combination with chromium and manganese improves sea-water resistance. Manganese is useful as a deoxidizer and desulfurizer in steel production and is useful for improving steel strength. In the present inventive steel more than 0.3% of manganese is necessary for the required strength, but too much manganese will cause embrittlement of the steel and is thus limited to 2.0%. Copper is useful for improving sea-water resistance. This effect cannot be expected with less than 0. 1% of copper, and more than 0.29% of copper will cause embrittlement and lowers weldability without any remarkable improvement in sea-water resistance. Therefore, the content of copper is limited to 0.1-0.29%. An excess amount of phosphorus embrittles the steel, and severely deteriorates joint portions of welded structures such as submarine pipe lines and buoys. Thus the upper limit of phosphorus is 0.04%. Sulphur, an unavoidable impurity, severely lowers sea-water resistance and weldability, and should be maintained as low as possible. A permissible upper limit of sulphur is 0.035%.

A second preferred composition according to the present invention comprises 0.001O.25% of carbon, 0.1- 2.0% of silicon, 03-20% of manganese, 0.01-0.5% of chromium (excluding 0.5%), 0.l0.29% of copper, 0.001-0.040% of phosphorus, 0.02-0.20% of two or more elements selected from the group consisting of arsenic, tin, beryllium, bismuth, lead, germanium, antimony, selenium, and tellurium, and the balance being iron and unavoidable impurities.

A third preferred composition according to the present invention comprises, in addition to the second type of preferred composition, one or more elements selected from the group consisting of titanium, zirconium, niobium, vanadium, molybdenum, tungsten, and nickel and in an amount of 0.010.50% in respect to titanium, zirconium, niobium, and vanadium alone or in; combination in; 0.1- 1.5% in respect to molybdenum, and tungsten in single or in combination; 0.13.5% in respect to nickel.

For the second and third preferred composition according to the present inventions, addition of the second group of alloying elements (titanium, zirconium, niobium, vanadium, molybdenum, nickel and tungsten) or addition of the first group of alloying elements (arsenic, antimony, tin, lead, selenium, tellurium, bismuth, beryllium and germanium) is useful to some degree for the desired corrosion resistance, but addition of these two groups of alloying elements in combination give remarkably improved and unique corrosion resistance which cannot be expected from the addition of either of the two groups alone.

The elements of the second group by themselves are effective to reduce hydrogen embrittlement, prevent intergranular corrosion and improve corrosion fatigue, but their effects are remarkably enhanced when added in combination of the elements of the first group. This is due to the fact that the elements of the first group lower the activity coeffcient of nitrides or carbides of elements of the second group to convert them into a useful state and give a strong protection against severe corrosion from the copresence of crude oil and sea-water, as well as remarkably reduce corrosion on the portions splashed with the seawater and corrosion by bacteria in the contaminated sea zone.

The addition of the first group of elements by themselves is effective to reduce corrosion due to their toxicity to bacteria. Their effects are increased two-fold when added in combination with the second group elements, and further a remarkably improved resistance against local corrosion is given by a unique effect which reduces the ability of corrosion products to form a cathode. Particularly, chromium and silicon used with the second group elements are useful for the above combined effect. Silicon and manganese used with the second group elements have a specific ability to remarkably reduce corrosion on the portions splashed with the sea-water or corrosion under the condition where the crude oil and the seawater are present, particularly when added in combination with copper and nickel, as in the third composition.

Therefore, in view of economy, productivity, and practical utility, silicon, chromium, manganese and copper are indispensable elements in the present invention. Nickel is elfective to give over-all resistance against corrosion 6 The present inventive steels having a composition as defined above may be used as hot rolled, as annealed or as tempered.

The present invention will be better understood from the following examples.

and improve toughness at low temperatures. As for carbon and phosphorus, they are necessary for desired work- Example (First yp inventive Steels) ability and weldability.

Th Second type of h present invsntive Steel has a Table 2 shows results of corrosion tests under similar basic composition comprising 0.0010.25% of carbon, sfirvice conditions as met y i y marine construc- 0 1. 2 0% f Silicon f manganese trons made on the present lnventlve steels and convenf chromium, 1 29% f copper, 001 04 f tional steels produced in a converter, cast into ingots phosphorus, the balance being iron, with addition of and hot rolled- I11 the table, in the column of 02 0 2 of two or more elements Selected f the Water resistance shows corrosion rates after rust refirst group of alloying elements to give excellent weld- 15 movfll in the sea'waterv Splashing Z0116 dete m1ined 011 ability. The reason for addition of two or more elements Speclmens of thlckness X loommt Wldth X 4000 of the first group of alloys is that although the addition of f length, pollshed all Surface Wlth N 160 emery, arsenic alone, for example, gives good corrosion resistlmmersed 2500 m the sea'water with 1500 ance, such element cannot be added in a large amount in Over the average Water level of ebb and flow for one view of weldability and mechanical properties. 0n the Y and Shows cormslon rates rust removal other .hand, with addition f a decreased amount of 1n the contaminated sea zone, determined on specimens senic together with another element of the group such as of thickness X 50 Width X 100 length, tin, improved weldability can be obtained without sacri- 190118115d on all Surface With 600 y, immersed fice of corrosion resistance. Thus, taking one considera- 1 belOW the Sea surface for one Yeali The corrosion tion with another, it is advantageous to add two or more rate is percent of the weight loss of the specimens to of the second group of alloying elements. that of an ordinary carbon steel.

TABLE 2 Sea-water resist- 81106 COH'OSlOI].

Chemical composition (percent) rate (percent) Number of specimens 0 Si Mn P 8 Cu Cr Ni A B Ordinary carbon steel.-- 18 .05 .61 .016 .023 .08 100 100 Conventional steel:

The third type of steel of the present invention further From Table 2, it is clear that the present inventive comprises, in addition to the above composition, one or steels show better corrosion resistance in either of the more elements selected from the second group of alloying corrosive circumstances than the conventional sea-water elements in amounts of 0.01-0.50% in respect to Ti, Zr, resistant steels.

Nb, and V, 0.11.5% in respect to M0 and W, and 0.1- Particularly in the contaminated sea zone, the present 3.5% in respect to Ni to give further improved corrosion inventive steel No. 4 which contains increased amount resistance and mechanical properties. The lower limits of of Si and Cr shows excellent corrosion resistance, and the above additional elements are shown as limits for in the sea-water splashing zone the present inventive tangible resistance against corrosion, while the upper limsteel No. 5 containing increased amount of Si and Mn its are shown as limits beyond which detrimental eifects With lowered chromium content show excellent corbegin to appear on steel-making, or mechanical properrosion resistance.

ties such as weldability and other practical properties.

For example, silicon is necessary for steel-making and Example 2! (Second and third yp of lnventlve Steels) in combination of chromium improves corrosion resistance. For this 0.1-2.0% of silicon is necessary. As for Table 3 Shows 1m11ar1y results of cofl'oslon tests on chromium, 0.0l-0.5% is necessary for similar eifects as 60 the pmfient lnventwe stee1s conventlonal steels P silicon. Copper content less than 0.09% gave no tangible duced a convelnter cast Into mgots P hot improvement in corrosion resistance while more than (A) In column of sea'water reslst'fmce 111 Table 3 f copper causes local corrosion and crackings and ShOWS COl'IOSlOH rates after rust removal 1n the sea-water lowers weldability. Regarding the carbon content, more Splashmg Zone determmed same Way as 111 than 0.3% of carbon embrittles the steel and lowers weld- Table and (B) Shows 90305101? rates after rust ability and corrosion resistance and further workability, mPVaI a Zone near confiammated sea zone deter while less than 0.001% of carbon lowers strength and remmed 1n the Sa me Way as (B) m Table (C) In Table quires a longer time of refining and deteriorates economy 3 Shows coP'oslon rates after F removal determlned and productivity. As for phosphorus, more than 0.04% on the Speclmens of thlckness X 50 Wldth of phosphorus embrittles the steel and lowers weldability X 100 length, PohShed on all surface WIT-h 320 remarkably.

The unavoidable impurities such as sulphur have severely detrimental effects on corrosion resistance and weldability, and thus should be maintained as low as possible, and should be limited up to 0.35%

emery and immersed horizontally for one month alternately in a tank filled with crude oil and then in a tank filled with the sea-water. As seen from the table, the present inventive steels show excellent corrosion resistance in any of the corrosion circumferences. 

